| Motor neuron pools form topographic maps of connectivity on the surface of some skeletal muscles. One such muscle, the serratus anterior (SA) muscle of the rat is innervated topographically by motor neurons projecting through ventral roots C6 and C7. This muscle displays a rostrocaudal topographic bias before birth, and the positional map is partially restored after denervation. Electrophysiological studies have shown that the influence of C6 gradually diminishes over progressively more caudal sectors, while that of C7 is steadily augmented. In the present study a morphological analysis was conducted to further examine the positional bias in the projection of the spinal motor pool onto the muscle surface; and to investigate whether muscle fiber type might contribute to the topographical distribution of nerve roots. An activity-dependent dye uptake method was adapted to map motor pools onto the surface of the SA muscle. Morphometric analysis of C6 and C7 terminals showed a rostrocaudal difference in their number and size. Thus, a morphological correlate of the topographic map was detected, consistent with our previous electro-physiological results. In addition, a rostrocaudal bias in fiber type distribution in the SA muscle led us to test the interaction of fiber type and position in determining the topography. Simply stated, could fiber type distribution explain the positional bias? When nerve terminals from different ventral roots (C6 or C7) were compared on fibers of a single type (type II), the position-dependent variation in their terminal size remained. However, this bias was lost when subtypes (type IIA, type IIB) were examined. Thus, we conclude that fiber type and position affect the size of terminals independently. In addition, analysis of type II fiber diameters that differed in their segmental origin showed a bimodal distribution from rostral to caudal sectors. Comparison of the subtype fiber diameters showed little variation rostrocaudally. From these results, we infer that C6 tends to innervate IIB fibers rostrally, but IIA fibers caudally; whereas C7 tends to innervate IIA fibers rostrally, but IIB fibers caudally. Thus, our results suggest that motor axons and muscle fibers bear complementary molecular labels. These labels may be co-regulated and vary with rostrocaudal position, and they may bias synapse formation in favor of positionally matched partners. |
